Impregnation of metals with silicon



Patented Jan. 3, 1939 UNITED STATES IMPBEGNATION F lllETALS WITH SILICON K. Ihrig, Milwaukee, Wis asslgnor to Globe Steel Tubes 00., Milwaukee, Wia, a corporation'of Delaware No Drawing. Application January 4, 1938, Serial No. 183,320

2 Claims.

This invention relates to the impregnation of metals with silicon, and especially to silicon .cementation of ferrous base articles.

It has been known for many years that iron 5 and steels containing substantial amounts of silicon are much more resistant to the action of certain corrosive media, especially mineral acids, and to scaling'at elevated temperatures, than in the absence of such silicon content. These properties of high silicon irons and steels have been made use of commercially in apparatus required to resist such influences. Despite their excellent corrosion resistance these alloys have suflered, however, from serious disadvantages. which have militated against their use and their more widespread adoption.

A major disadvantage arises from the fact that for adequate resistance to attack of, for example, strong acids the content of silicon must be rather high, say above about '7 per cent, and most suitably even higher, a well known iron alloy of this class containing about 14 per cent of silicon. Unfortunately, such alloys are extremely brittle so that they are quite susceptible to breakage by slight shock or sudden changes of temperature.

Additionally, it is ordinarily not possible or economical to machine castings made from these alloys, for which reason any finishingmust ordinarily be done bygrinding. The restriction to use in the cast state, the difficulties in finishing the castings for use, and the liability to ready breakage, with their attendant cost burdens have retarded the wide-spread use of these otherwise desirable iron-silicon alloys.

It has been proposed to avoid the foregoing disadvantage by cementation of metallic. articles with silicon to provide a case, or coating, possessing the desired surface characteristics. For instance, it has been proposed to pack iron and 0 steel articles in powdered silicon or ferro-silicon and subject them to heat in much the same manner as case carburizing is effected. Various other modes of achieving the same end have been suggested. As far as I am aware, however, such proposals have been generally incapable of and unsuited for commercial application, because cementation can not be effected or the production of cementation requires such high temperatures or such long exposures, or'both, as to be commercially impracticable, or because the cases produced are not adapted to commercial requirements, due, for instance, to poor adherence, extreme thinness, or low silicon content, which characteristics render them of low or no particular value.

In my copending application Serial No. 86,823, filed June 23, 1936, now Reissue Patent No. 20,719, granted May 10, 1938, I have disclosed and claimed a method 01' impregnating metals with silicon which overcomes the disadvantages here- 5 tofore encountered in the art, not only those attendant upon the production and use of articles consisting throughout of high silicon alloys, but also those encountered in previous attempts to effect silicon cementation. Briefly, in accordin ance with that invention cementation with silicon is eflected by heating the article to an elevated temperature, and while so heated contacting it with a reagent formed by heating a silicon-bearing material, such as elemental silicon, silicon 15 carbide and ferro-silicon, in a current of a reagent of the group chlorine gas and chloride vapor.

In thepreferred embodiment of that invention the article is packed in the silicon-bearing material, and is heated under non-oxidizing condi- 20 tions, to the cementing temperature, and then a current of chlorine gas is passed into the container, the heating and introduction of chlorine being continued for a time to cause the silicon to impregnate the article to the desired depth. 5 The cementing reaction occurs as low as about 1300 F. with ferro-silicon, while with silicon carbide it begins at about 1500 F.; desirably, however, the temperature is above about 1600 F., most suitably about 1800 to 1850" F. Altema- 30 tively the article is heated in a container, and when at cementing temperature there is introduced siliconizing reagent produced by treating the silicon-bearing material with chlorine or chloride vapor in a separate container; in this 35 embodiment the article may be cemented at substantially the same temperature as in the packing method, but higher temperatures, say of the order of 2300 F., are required to form the siliconizing reagent. 40

Through the practice of the invention of that application there are produced articles having highly satisfactory silicon-rich cases which are coherent and satisfactorily adherent to the underlying core, as well as being resistant to acid 5 and oxidizing attack. They may be made of desired thickness, and, in fact, articles may be impregnated throughout after being formed by rolling, forging, or the like. That method is thus productive of commercially desirable prod- 50 nets, and it is capable of satisfactory use. In fact, it is now, and for some time past has been, in commercial operation, and interest in its commercial adoption and expansion to new uses continues to increase rapidly. 55

It is characteristic, as far as I have been able to discover, of the method disclosed in my copending application that the cases produced contained from about 12 to about 14 per cent of silicon, with the maximum silicon content at approximately 14.3 per cent. It is known that variation in silicon content of ferrous base alloys of a few tenths of l per cent may cause substantial change in the corrosion resistance, especially above about 7 per cent of silicon, and with the maximum resistance to acid attack at about 14 per cent of silicon it would be desirable, for some purposes, to provide cases embodying the desirable characteristics of but containing a higher concentration of silicon than is present in those produced in regular commercial practice of my aforesaid invention.

It is among the objects of this invention to increase the thickness and silicon content of siliconcontaining cases preformed on metallic articles, to provide thereby a higher silicon content in such cases than is attainable by the procedure used in forming the case initially, and by a procedure which is cheap, easily practiced, and does not require complicated or expensive apparatus. A particular object of the invention is to achieve the foregoing objects by the use of silicon or ferro-silicon as the sole reagent. Other objects of the invention will appear from the following specification.

For ease of reference the invention will be described in connection with the treatment of ferrous base alloys although it will be understood that it is applicable also to the treatment of articles formed from other metals capable of impregnation by silicon. Silicon and ferro-silicon are themselves without appreciable cementing ef fect upon ferrous base articles at practically applicable temperatures and commercially feasible periods of exposure. I have discovered, however, and it is upon this that my invention is predicated in part, that if ferrous base articles are first cemented, or impregnated with silicon in their marginal layers, the concentration of silicon in and the thickness of such case may be achieved readily, simply and relatively quickly by simple heating of the pre-cased articles in contact with an agent containing silicon.

The invention is not restricted to the treatment of articles pre-cased in any particular manner but rather appears to be applicable generally to increasing the thickness and silicon content of silicon-bearing cases produced in any manner whatsoever. Moreover, the benefits of the invention are attained even though the initial cementation be of quite incipient character, i. e., to an exceedingly slight depth. In this manner cases of commercially desirable depth and silicon content can be produced by procedures otherwise un feasible, and the concentrations produced by the method of my aforesaid application are increased Without detriment to the other properties of the cases.

As silicon-supplying reagents applicable in the practice of the present invention, there may be used elemental silicon, suitably in its commercially pure forms, or ferro-silicons containing substantial amounts of silicon, and other similar materials. Silicon carbide, in which the silicon is apparently largely or completely in combined form, is not such a silicon-supplying reagent as is applicable in the practice of this invention.

The invention may be illustrated by reference to actual tests which exemplify its benefits. In these tests SAE 1015 steel was used but it will be understood that the invention is not restricted thereto. Bars of this steel in the form of inch rounds were treated preliminarily in accordance with the method of my above-identified application, by heating them to about 1800 F. in a rotary container in contact with silicon carbide, and then introducing chlorine into the container, to produce silicon-rich cases of substantial thickness. Treatedanduntreatedbarswerethenplaced with ferro-silicon in a rotary container and heated in accordance with the present invention. In one such test the case produced in the preliminary treatment contained 13.08 per cent of silicon. After being rotated for 2 hours at 1850 F. in ferro-silicon containing 30 per cent of silicon, the silicon in the case was increased to 13.19 per cent. In another similar test, the case on a similarly pre-treated bar contained 13.28 per cent of silicon, which was increased to 13.76 per cent by heating the bar, in similar manner, for 2 hours at 1900 F. with the same grade of ferro-silicon. In both instances bars not previously cased were unaffected by the treatment.

Generally speaking, it appears that the higher the percentage of silicon in the reagent, the lower the temperature at which there are produced the results which characterize this invention, and the greater the relative increase in silicon content. For instance, bars of the SAE 1015 steel pretreated in the same manner were heated for 2 hours at 1650 F. in a rotary furnace with ferrosilicon containng 46 per cent of silicon. Thereby the silicon content of the case was increased from 12.42 per cent in the original case to 13.71 per cent. Another bar whose case initially contained 13.68 per cent was heated similarly for the same time in the same grade of ferro-silicon at 1750 F., after which the case contained 14.45 per cent of silicon, which, by comparison with the example just given, illustrates the benefits to be derived from the use of higher temperatures. Bars not pre-cased were not impregnated with silicon in either of these tests; i. e., they were not affected by the treatment of this invention. Comparison of the results obtained in these two tests with those cited in the next preceding paragraph shows how the use of silicon-rich ferro-siliccn confers greater increase in silicon content even at lower temperatures.

Similar results have been obtained using commercially pure silicon containing about 3 per cent of impurities. On the other hand, using ferrosilicon containing about 16 per cent of silicon I have thus far been unable to produce any in crease in silicon content at temperatures as high as 1900" F., showing that materials of this type must contain substantial amounts of silicon, e. g., in excess of the amount required by the formula FeaSi.

In the practice of the invention the article may be packed in a box of the type used generally for cementation operations, or it may beplaced in a rotary container together with an amount of reagent sufficient to cause the article to be in contact therewith while the container is rotated during the treatment. With some types of articles, such as tubes, the rotary, or tumbling, procedure is desirable. The temperature should be at least about 1300 F., but for most purposes higher tem-- peratures, say 1650 to 1850 or 1900 F. are desirable. In general it seems that to obtain similar results lower temperatures may be used in the pack than in the tumbling procedure.

I have discovered furthermore that the benefits of the invention are particularly attainable through a special combination of the procedure of this invention with that described in my aboveidentified application. More particularly, I have discovered that if in the use of the invention of that application the flow of chlorine or chloride vapor, or of reagent gas formed extraneously, be discontinued and ferro-silicon or silicon be then introduced into the treating chamber and the heating continued, it is possible not only to increase substantially the silicon content and thickness of the previously formed case as described above, but also that it is possible to produce cases of substantial thickness and desirable silicon concentration on untreated articles introduced with the silicon or ferro-silicon.

The tests now to be described were performed likewise upon /z-inch round bars of SAE 1015 steel. In one test such bars were placed in a rotatable retort together with 30 pounds of silicon carbide, and the materials were heated to about 1800 F. and then, while continuing the heating, 4.5 pounds of chlorine were introduced during about 2 hours. The current of chlorine was then turned off, 5 pounds of ferro-silicon were added and at the same time there was introduced a bar previously cased to provide it with a low silicon content, namely, 11.32 per cent. The container and its contents were then heated to 1830 F. for 1 hour, after which time the silicon content of this bar was 13.7 per cent.

In another test similar bars were cased by rotation with 30 pounds of silicon carbide for 2 hours at about 1800 F. while 5.25 pounds of chlorine were introduced. The current of chlorine was then shut ofi, 5 pounds of ferro-silicon were added together with a bar containing a very thin case produced by 15minutes treatment in accordance with my above-identified application. The container and its contents were heated 1 hour at 1840 F, which treatment increased the case on the bar from 0.003 inch thickness to 0.03 inch, and its silicon content to 14.7 per cent, a very high value indeed. The production of cases containing such amounts of silicon is important in achieving maximum resistance to oxidation and corrosion.

In still another test articles were treated in accordance with my prior invention by rotating them at about 1800 F. for 2 hours in contact with 30 pounds of silicon carbide while introducing 4.75 pounds of chlorine. The chlorine was then shut oh, and 5 pounds of ferro-silicon were added to the container together with two bars of steel:- Bar A was similar to that used in the immediately preceding example, having a case 0.003 inch thick of relatively lowsilicon content, while bar B had not been previously cemented. After the container had been heated for 1 hour at 1840 F. each bar had a case 0.03 inch thick and the silicon content ofbar A was increased to 14.33 per cent, while the previously untreated bar (B) had a case containing 14.47 per cent of silicon.

In all of these latter tests the silicon content of the articles cased by the preliminary treatment, according to the method of my aforesaid application, was increased.

This last test evidences the applicability of the invention to the production of thick highsilicon cases .upon previously untreated materials where the present invention is coupled in the manner described above with the method described in my copending application. While I do not limit myself to this explanation, I now believe that the ability to produce such cases upon untreated materials is due to the efiect of some residual chlorine or chloride in the treating chamber at the time when the ferro-silicon and the untreated articles are added and which acts to form an incipient case whose thickness and silicon content are then increased by the ferrosilicon in accordance with the present invention. Be that as it may, it is demonstratable that where untreated articles are merely contacted with fresh ferro-silicon or silicon it is not possible to produce cases like those which result from the practice of the present invention.

From the fact that the reagents used, such as silicon and ferro-silicon, alone suflice to enrich the silicon content of pre-formed silicon cases, it would appear that these operable agents supply the silicon in elemental form, although I do not confine myself to that theory. Similarly, the inoperability of silicon carbide and of ferro-silioon containing less than about 16 per cent of silicon, make it appear that for the purposes of the invention the reagent must contain silicon in uncombined or free form. However, other factors may be the true explanation of these things. For these reasons the terms silicon-supplying material and agent supplying silicon to the case used in the claims to comprehend those reagents which when used in accordance with this invention serve to accomplish its objects.

It may be noted that silicon and ferro-silicon act as silicon-supplying agents while in the solid state.

According to the provisions of the patent statutes, I have explained the principle of my invention and have illustrated and described what I now consider to represent its best embodiment. However, I desire to have it understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically illustrated and described.

I claim:

1. That method of making silicon-cemented ferrous base articles which comprises heating the article to an elevated temperature and contacting it with siliconizing reagent formed by heating a member of the group silicon, silicon carbide and ferro-silicon in a current of a member of the group chlorine gas and chloride vapor, continuing such heating and treatment for a time to effect siliconizing of at least the marginal layers of said article, then discontinuing such reagent and adding a silicon-supplying agent and continuing heating to a temperature of at least about 1300 F. and thereby increasing the concentration of silicon in the initially formed case.

2. That*-method of making silicon-cemented ferrous base articles which comprises heating the article to at least about 1300 F. and contacting it with siliconizing reagent formed by heating a member of the group silicon, silicon carbide and ferro-silicon in a current or a member of the group chlorine gas and chloride vapor, continuing such heating and treatment for a time to. eflect siliconizing at least in the marginal layers of said article, then discontinuing such reagent and adding an agent of the group silicon and ferro-silicon' 

